Lens system and projector utilizing the same

ABSTRACT

A high-performance lens system including lens elements of small diameters for projecting on to a screen or the like in an enlarged fashion images from light valves such as mainly DMDs (Digital Micromirror Devices) for forming images by changing reflecting directions of light is provided. 
     A lens system includes, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, the second lens group being configured to include, sequentially in that order from a magnifying side thereof, a first sub-lens group or 2a lens group having a negative refractive power as a whole and a second sub-lens group or 2b lens group having a positive refractive power as a whole, and depending upon applications, a third lens group made up of a single positive lens element is provided in the vicinity of a light valve such as a DMD on a contracting side of the second lens group.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-334018, filed on Dec. 12, 2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens system including lens elements of small diameters for projecting on to a screen in an enlarged fashion images from light valves such as mainly DMDs (Digital Micromirror Devices) for forming images by changing reflecting directions of light.

2. Description of the Related Art

Projectors utilizing DMDs as light valves are considered as being more advantageous in miniaturization than those utilizing other image forming methods. Currently, compact portable projectors have been well accepted by many users, and especially so are data projectors which are convenient for presentations.

In projectors which are developed for use as portable ones, reducing the thickness dimension is of importance, and it can be said that the reduction in thickness is one of the most important factors for projectors which are in many cases carried together with note-type personal computers or the like for combined use. As an example of a means for solving the problem, for example, the Japanese Unexamined Patent Publication No. 2003-121736 describes a design method for reducing the effective diameter of a projection lens.

SUMMARY OF THE INVENTION

The present invention has been made in view of the situations and an objective thereof is to realize a lens system made up of lens elements of small diameters, so as to provide a projector incorporating the lens system so realized which is thus thin enough for convenient carriage and which can project an enlarged image with high image quality even in a limited space.

According to a preferred aspect of the invention, there is provided a lens system including, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole.

The first lens group is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1a lens group having a negative refractive power as a whole and a second sub-lens group or 1b lens group having a positive refractive power as a whole, and the second lens group is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2a lens group having a negative refractive power as a whole and a second sub-lens group or 2b lens group having a positive refractive power as a whole.

In addition, this lens system is such that the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the 1a lens group, the following conditional expression (4) is satisfied with respect to a power set for the 2a lens group, the following conditional expression (5) is satisfied with respect to a power set for the 2b lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group;

−0.5≦f/f _(I)≦−0.1  (1)

0.3≦h _(IE) /h _(IX)≦0.5  (2)

−1.5≦f/f _(Ia)≦−0.5  (3)

−0.3≦f/f _(IIa)≦0.3  (4)

0.15≦f/f _(IIb)≦0.7  (5)

1.6≦T _(I) /f≦2.6  (6)

where, f: Composite focal length of the whole lens system; f_(I): Composite focal length of the first lens group; h_(IE): Height of paraxial ray incident on to a magnifying-side surface of a lens element which is disposed outermost on a magnifying side of the first lens group; h_(IX): Height of paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f_(Ia): Composite focal length of the 1a lens group which makes up the first lens group; f_(IIa): Composite focal length of the 2a lens group which makes up the second lens group; f_(I): Composite focal length of the 2b lens group which makes up the second lens group; and T_(I): Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the configuration of lens elements of a lens system according to a first embodiment of the invention,

FIG. 2 is a drawing showing aberrations produced by the lens system according to the first embodiment;

FIG. 3 is a drawing showing the configuration of lens elements of a lens system according to a second embodiment of the invention,

FIG. 4 is a drawing showing aberrations produced by the lens system according to the second embodiment;

FIG. 5 is a drawing showing the configuration of lens elements of a lens system according to a third embodiment of the invention,

FIG. 6 is a drawing showing aberrations produced by the lens system according to the third embodiment;

FIG. 7 is a drawing showing the configuration of lens elements of a lens system according to a fourth embodiment of the invention,

FIG. 8 is a drawing showing aberrations produced by the lens system according to the fourth embodiment;

FIG. 9 is a drawing showing the configuration of lens elements of a lens system according to a fifth embodiment of the invention,

FIG. 10 is a drawing showing aberrations produced by the lens system according to the fifth embodiment;

FIG. 11 is a drawing showing the configuration of lens elements of a lens system according to a sixth embodiment of the invention,

FIG. 12 is a drawing showing aberrations produced by the lens system according to the sixth embodiment;

FIG. 13 is a drawing showing the configuration of lens elements of a lens system according to a seventh embodiment of the invention,

FIG. 14 is a drawing showing aberrations produced by the lens system according to the seventh embodiment;

FIG. 15 is a drawing showing the configuration of lens elements of a lens system according to an eighth embodiment of the invention,

FIG. 16 is a drawing showing aberrations produced by the lens system according to the eighth embodiment;

FIG. 17 is a drawing showing an external appearance of a projector which utilizes the lens system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described by reference to specific numerical examples. In Embodiments 1 to 8 below, a lens system is configured to include, sequentially in that order from a magnifying side, a first lens group LG1 which makes up a substantially afocal optical system as a whole and a second lens group LG2 having a positive refractive power as a whole.

The first lens group LG1 is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1a lens group LG1 a having a negative refractive power as a whole and a second sub-lens group or 1b lens group LG1 b having a positive refractive power as a whole.

The 1a lens group LG1 a is configured to include three lens elements which are, sequentially in that order from a magnifying side, a lens element (lens element designated as L111, a magnifying-side surface as 111, a contracting-side surface as 112) which has a positive refractive power (hereinafter, referred to as a positive lens element), a lens element (lens element designated as L112, a magnifying-side surface as 113, a contracting-side surface as 114) which is formed into a meniscus shape which is convex on a magnifying side thereof and which has a negative refractive power (hereinafter, referred to as a negative lens element), and a negative lens element (lens element designated as L113, a magnifying-side surface as 115, a contracting-side surface as 116).

The 1b lens group LG1 b, which follows the 1a lens group LG1 a, is configured to include four lens elements in total, that is, a negative lens element (lens element designated as L121, a magnifying-side surface as 121, a contracting-side surface as 122), a positive lens element (lens element designated as L122, a magnifying-side surface as 123, a contracting-side surface as 124, however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 122 and the lens surface 123 constitute the same lens surface), a negative lens element (lens element designated as L123, a magnifying-side surface as 125, a contracting-side surface as 126), and a positive lens element (lens element designated as L124, a magnifying-side surface as 127, a contracting-side surface as 128) which has a meniscus shape which is convex on a contracting side thereof, or is configured to include five lens elements in total, that is, a negative lens element (lens element designated as L121, a magnifying-side surface as 121, a contracting-side surface as 122), a positive lens element (lens element designated as L122, a magnifying-side surface as 123, a contracting-side surface as 124, however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 122 and the lens surface 123 constitute the same lens surface), a negative lens element (lens element designated as L123, a magnifying-side surface as 125, a contracting-side surface as 126), a positive lens element (designated as L124, a magnifying-side surface as 127, a contracting-side surface as 128), and a positive lens element which has a meniscus shape which is convex on a contracting side thereof or a negative lens element which has a meniscus shape which is convex on a contracting side thereof (lens element designated as L125, a magnifying-side surface as 129, a contracting-side surface as 1210).

The second lens group LG2 is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2a lens group LG2 a having a negative refractive power as a whole and a second sub-lens group or 2b lens group LG2 b having a positive refractive power as a whole.

The 2a lens group LG2 a is configured to include three lens elements, that is, a positive lens element (lens element designated as L211, a magnifying-side surface as 211, a contracting-side surface as 212), a positive lens element (designated as L212, a magnifying-side surface as 213, a contracting-side surface as 214), and a negative lens element (lens element designated as L213, a magnifying-side surface as 215, a contracting-side surface as 216, however, in the event that the lens element constitutes a cemented lens element together with the preceding positive lens element, the lens surface 214 and the lens surface 215 constitute the same lens surface).

The 2b lens group LG2 b is configured to include three lens elements in total, that is, a positive lens element (lens element designated as L221, a magnifying-side surface as 221, a contracting-side surface as 222), a negative lens element (lens element designated as L222, a magnifying-side surface as 223, a contracting-side surface as 224), and a positive lens element (lens element designated as L223, a magnifying-side surface as 225, a contracting-side surface as 226, however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 224 and the lens surface 225 constitute the same lens surface).

Additionally, the 1a lens group LG1 a and the 1b lens group LG1 b, which make up the first lens group LG1, are fixed to a first lens barrel, and the 2a lens group LG2 a and the 2b lens group LG2 b, which make up the second lens group LG2, are fixed to a second lens barrel.

In addition, depending upon applications, the lens system is configured to include a third lens group LG3 which is made up of a positive lens element (lens element designated as L301, a magnifying-side surface designated as 301, a contracting-side surface designated as 302) which is disposed in the vicinity of a light valve such as a DMD on the contracting side of the second lens group LG2. A cover glass CG (a magnifying-side surface as C01, a contracting-side surface as C02), which is a constituent component of the light valve such as the DMD, is provided between a contracting side of the third lens group LG3 and a surface of the light valve with a slight airspace provided therebetween.

In this way, according to the embodiment, there is provided a lens system comprising, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, the first lens group being configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1a lens group having a negative refractive power as a whole and a second sub-lens group or 1b lens group having a positive refractive power as a whole, the second lens group being configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2a lens group having a negative refractive power as a whole and a second sub-lens group or 2b lens group having a positive refractive power as a whole, wherein, the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the 1a lens group, the following conditional expression (4) is satisfied with respect to a power set for the 2a lens group, the following conditional expression (5) is satisfied with respect to a power set for the 2b lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group;

−0.5≦f/f _(I)≦−0.1  (1)

0.3≦h _(IE) /h _(IX)≦0.5  (2)

−1.5≦f/f _(Ia)≦−0.5  (3)

−0.3≦f/f _(IIa)≦0.3  (4)

0.15≦f/f _(IIb)≦0.7  (5)

1.6≦T _(I) /f≦2.6  (6)

where, f: Composite focal length of the whole lens system; f_(I): Composite focal length of the first lens group; h_(IE): Height of paraxial ray incident on to a magnifying-side surface of a lens element which is disposed outermost on a magnifying side of the first lens group; h_(IX): Height of paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f_(Ia): Composite focal length of the 1a lens group which makes up the first lens group; f_(IIa): Composite focal length of the 2a lens group which makes up the second lens group; f_(IIb): Composite focal length of the 2b lens group which makes up the second lens group; and T_(I): Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.

The conditional expression (1) specifies an appropriate power distribution to the first lens group which makes up the substantially afocal optical system as a whole. In the event that an upper limit is surpassed, the positive power of the first lens group becomes too large, and the back focus becomes short. In the event that a lower limit is surpassed, the negative power of the first lens group becomes too large, and the load borne by the rear group increases, whereby the aberrations are deteriorated.

In addition, the conditional expression (2) specifies the magnification of the first lens group which functions as a wide converter, and in the event that an upper limit thereof is surpassed, the magnification becomes too high, whereby the load in terms of aberrations borne by the front group becomes too large, resulting in a deteriorated performance, whereas a lower limit is surpassed, the magnification decreases, which is disadvantageous in wide-angle setting.

The conditional expression (3) is associated with the negative power set for the 1a lens group which corresponds to a dispersion system in the first lens group which makes up the wide converter. In the event that an upper limit is surpassed, the negative power becomes weak, whereby the focal length of the whole converter system is increased, and the system has to be enlarged. In the event that a lower limit is surpassed, the negative power becomes too large, which is advantageous in miniaturization but is disadvantageous in correcting the aberrations.

The following conditional expressions (4), (5) are associated with the power configuration of the second lens group. Although the second lens group, which makes up the rear group, has the positive power as the whole system, the second lens group is characterized in that since the 2a lens group, which is situated on the magnifying side, constitutes the dispersion system, the 2a lens group is configured to have the negative power as a whole, and since the 2b lens group, which is situated on the contracting side, constitutes a condenser system, the 2b lens group is configured to have the positive power as a whole.

The conditional expression (4) specifies the negative power of the 2a lens group, and in the event that an upper limit is surpassed, the negative power becomes small, whereby the back focus is shortened. On the contrary, in the event that a lower limit is surpassed, the negative power becomes too large, giving rise to enlargement of the system, and the aberrations are increased by the negative power which now becomes excessive.

The conditional expression (5), which follows the expression (4), specifies the positive power of the 2b lens group so as to obtain an appropriate size and performance for the whole lens system together with the conditional expression (4). In the event that an upper limit is surpassed, the positive power becomes immoderate, and the performance is decreased and the back focus is shortened. In the event that a lower limit is surpassed, the positive power becomes small, giving rise to enlargement of the system.

The conditional expression (6) specifies the thickness of the first lens group in the optical axis direction. Functioning as the afocal converter, the first lens group needs to keep a certain axial thickness for correction of the aberrations, although it depends upon the afocal magnification thereof. In the event that an upper limit is surpassed, although it is advantageous in terms of performance, the system has to be enlarged, whereas in the event that a lower limit is surpassed, although it is advantageous in terms of miniaturization, the powers of the 1a lens group and the 1b lens group become too large, whereby the aberrations are increased.

In addition, the 1a lens group, which makes up the first lens group, is configured to include three lens elements, sequentially in that order from a magnifying side, a positive lens element, a negative lens element which has a meniscus shape which is convex on a magnifying side thereof, and a negative lens element, and it is preferable that the following conditional expression (7) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1a lens group, and the following conditional expression (8) is satisfied with respect to the refractive index of a glass material used for each of the lens elements which make up the 1a lens group;

0.6≦R _(Ia4) /f≦1.1  (7)

1.65≦N_(Ia)  (8)

where,

R_(Ia4): Radius of curvature of the contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1a lens group; and N_(Ia): Mean value of refractive indices of the lens elements which make up the 1a lens group relative to the d line.

The conditional expression (7) is associated with the shape of the contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1a lens group, and this lens element plays an important role due to the lens element being disposed outermost on the magnifying side as a negative lens in the lens group having the negative power. Namely, the contracting-side surface of the relevant lens element is formed substantially into a concentric shape relative to a bundle of rays on the magnifying side while being made to have a strong power, and basically, the concentric shape so formed is made to suppress the generation of aberrations. Consequently, in the event that a lower limit is surpassed, spherical aberration and comatic aberration are overcorrected, whereas in the event that an upper limit is surpassed, they become undercorrected, on the contrary.

The conditional expression (8) is associated with the characteristics of the refractive index of the 1a lens group which has the particularly strong negative power. In order to mitigate the intensity of curvature of acquiring the strong negative power, it is inevitable that the refractive index is as high as possible, and in the event that a lower limit of the conditional expression is surpassed, the curvature becomes excessive, whereby the spherical aberration and the comatic aberration become excessive, and the Petzval sum becomes too small, thereby making it impossible to obtain a good performance.

In addition, the 1b lens group is configured to include four lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof, or is configured to include five lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, a positive lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof or a negative lens element which has a meniscus shape which is convex on a contracting side thereof, and it is preferable that the following conditional expression (9) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the contracting side of the 1b lens group, the following conditional expression (10) is satisfied with respect to the dispersion properties of a glass material used for each lens element which makes up the 1b lens group, and the following conditional expression (11) is satisfied with respect to the refractive index of a glass material used for each of the negative lens elements which make up the 1b lens group;

−2.8≦R _(IbL) /f≦−1.0  (9)

V _(IbP) −V _(IbN)≦−5  (10)

1.58≦N_(IbN)  (11)

where,

R_(IbL): Radius of curvature of the contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 1b lens group; V_(Ibp): Mean value of Abbe numbers of the positive lens elements which make up the 1b lens group; V_(IbN): Mean value of Abbe numbers of the negative lens elements which make up the 1b lens group; and N_(IbN): Mean value of the refractive indices of the negative lens elements which make up the 1b lens group relative the d line.

The conditional expression (9) is associated with the shape of the lens element which is disposed outermost on the contracting side of the 1b lens group, and the shape has a role of transmitting a bundle of dispersed rays from the 1a lens group to the rear while maintaining the generation of aberrations to a low level. In the event that an upper limit is surpassed, the correction of a large negative distortion produced by the 1b lens group becomes insufficient, whereas in the event that a lower limit is surpassed, although it is advantageous in correcting the distortion, the meniscus shape becomes too intensive, and high-order spherical aberration and comatic aberration are generated.

The conditional expression (10) is associated with the achromatism of the 1b lens group and specifies an appropriate combination of Abbe numbers in the glass material. In the event that a lower limit is surpassed, it becomes difficult to correct chromatic aberration.

The conditional expression (11) specifies a restricting condition for correction of field curvature, spherical aberration and comatic aberration. Since the Petzval sum tends to become negative easily, it is good to correct it by increasing the refractive indices of the negative lens elements, and together with this, the correction of spherical aberration and comatic aberration is intended by increasing the radius of curvature by the high refractive indices. Due to this, in the event that a lower limit is surpassed, it becomes difficult to correct field curvature.

In addition, the 2a lens group is configured to include three lens elements, that is, a positive lens element, a positive lens element and a negative lens element, and it is preferable that the following conditional expressions (12), (13) are satisfied, respectively, with respect to the refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2a lens group, the following conditional expression (14) is satisfied with respect to a power set for the negative lens element which is disposed outermost on the contracting side of the 2a lens group, and the following conditional expression (15) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2a lens group and the shape of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the same lens group;

0.15≦N _(IIaN) −N _(IIaP)  (12)

10≦V _(IIaP) −V _(IIaN)  (13)

−1.5≦f/f _(IIaN)≦−0.4  (14)

−0.75≦R _(IIa2) /R _(IIa3)≦−0.1  (15)

where,

N_(IIaN): Mean value of refractive indices of the negative lens element which makes up the 2a lens group relative to the d line; N_(IIaP): Mean value of refractive indices of the positive lens elements which make up the 2a lens group relative to the d line; V_(IIaP): Mean value of Abbe numbers of the positive lens elements which make up the 2a lens group; V_(IIaN): Mean value of Abbe numbers of the negative lens elements which make up the 2a lens group;

f_(IIaN): Focal length of the lens element which is disposed outermost on the contracting side of the lens elements which make up the 2a lens group;

R_(IIa2): Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the lens elements which make up the 2a lens group; and R_(IIa3): Radius of curvature of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the lens elements which make up the 2a lens group.

The conditional expression (12) specifies a condition for field curvature correction, which corrects a field curvature produced by the strong negative power of the 2a lens group. Namely, it is good to use a material having a high refractive index for the negative lens element and a material having a low refractive index for the positive lens element, so as to prevent the Petzval sum from becoming too small. In the event that a lower limit is surpassed, the Petzval sum becomes too small, whereby the field curvature is worsened.

The conditional expression (13) is a condition for correction of chromatic aberration of the 2a lens group. In the event that an upper limit is surpassed, the Abbe numbers come nearer to each other, and the chromatic aberration effect becomes insufficient.

The conditional expression (14) is, as with the conditional expression (13), a condition for correction of chromatic aberration of the 2a lens group, as well. As is known, since chromatic aberration can be corrected by an appropriate combination of Abbe number and power, the conditional expression (14) specifies the power. In the event that an upper limit and a lower limit are surpassed, in both cases, the achromatic power of the 2a lens group becomes inappropriate, whereby the chromatic aberration are worsened.

The conditional expression (15) is associated with correction of spherical aberration and comatic aberration within the 2a lens group. The correction of those aberrations can be enabled by designing the shape of the convex lens element within the 2a lens group in such a manner as to fall within a range restricted by the conditional expression (15). In the event that a lower limit is surpassed, the spherical aberration becomes over, and the comatic aberration relative to a bundle of off-axis rays is also worsened. In the event that an upper limit is surpassed, the spherical aberration becomes under excessively.

In addition, the 2b lens group is configured to include three lens elements in total, that is, a positive lens element, a negative lens element, and a positive lens element, and it is preferable that the following conditional expressions (16), (17) are satisfied with respect to refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2b lens group, respectively, the following conditional expression (18) is satisfied with respect to a power set for the lens element which is disposed second outermost from the magnifying side of the 2b lens group, and the following conditional expression (19) is satisfied with respect to the shape of a magnifying-side surface and the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group;

0.15≦N _(IIbN) −N _(IIbP)  (16)

0≦V _(IIbP) −V _(IIbN)  (17)

−1.0≦f/f _(IIb2)≦−0.2  (18)

−2.0≦R _(IIb1) /R _(IIb2)≦−0.3  (19)

where,

N_(IIbN): Mean value of refractive indices of the negative lens element which makes up the 2b lens group relative to the d line; N_(IIbP): Mean value of refractive indices of the positive lens elements which make up the 2b lens group relative to the d line; V_(IIbP): Mean value of Abbe numbers of the positive lens elements which make up the 2b lens group; V_(IIbN): Mean value of Abbe numbers of the negative lens element which makes up the 2b lens group;

f_(IIb2): Focal length of the lens element which lies second outermost from the magnifying side of the lens elements which make up the 2b lens group;

R_(IIb1): Radius of curvature of a magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group; and R_(IIb2): Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group.

The conditional expression (16) is associated with the refractive index of a glass material for each of the lens elements which make up the 2b lens group which makes up the second lens group and has a large positive power. By giving a difference in refractive index between the positive and negative lens elements, a correcting capability of spherical aberration is made use of on a cemented surface while maintaining the generation of chromatic aberration to a minimum level, and the effect of field curvature correction is also expected. In the event that a lower limit is surpassed in the conditional expression (16), it results in overcorrection of field curvature and results in undercorrection of spherical aberration.

The conditional expression (17) specifies a condition for color correction in the 2b lens group, and in order to correct single-color aberrations at the same time, it is necessary that the powers of the respective lens elements do not become excessive, and to make this happen, it is a necessary condition that Abbe numbers of glass materials used for the positive lens elements and the negative lens element satisfy the conditional expression (17). In the event that a lower limit is surpassed, it becomes difficult to correct chromatic aberration.

The conditional expression (18) similarly specifies a condition for chromatic aberration correction or achromatism of the 2b lens group. In the event that both an upper limit and a lower limit are surpassed, in both cases, the achromatic power in the 2b lens group becomes inappropriate, and the chromatic aberration is worsened.

The conditional expression (19) specifies a restricting condition for the shape of the lens element which is disposed outermost on the magnifying side of the 2b lens group and is associated with spherical aberration and comatic aberration. In the event that an upper limit is surpassed, the spherical aberration becomes under largely, whereas in the event that a lower limit is surpassed, the generation of comatic flare in a bundle of marginal rays becomes remarkable.

In addition, as a method for adjusting the focal point of the whole lens system, a method for shifting the whole projection lens in the optical axis direction cannot be said to be a good method in a case where there is imposed a limitation on the size of a structure to be shifted. Then, in adopting a so-called inner focusing method in which the focal point is adjusted by shifting part of an optical system of a lens system, when taking it into consideration that the first lens group is substantially afocal, it is considered to make use of the lens group which is disposed closer to the contracting side than the first lens group, and it is preferable to adjust the focal point by shifting the second lens group in the optical axis direction because a spatial amount for shifting is sufficient and the change in aberrations due to distance can be suppressed to a least level.

Furthermore, in order to focus bundles of rays from the DMDs on to the surface of a screen with good efficiency, the angle of a principal ray of a bundle of rays emerging from the DMD must be set according to the properties of the illuminating optical system. Due to this, the lens system as being a projection lens is set substantially telecentric in many cases. However, when attempting to design such that all the rays are captured, the effective diameter of the second lens group on the contracting side thereof becomes gigantic, which in many cases causes a problem with the arrangement of the second lens group relative to the illuminating optical system. As this occurs, although there is imposed a limitation on the effective diameter in the vicinity of the emerging side of the second lens group depending upon the arrangement of the second lens group with the illuminating optical system, the objective is preferably attained by providing a third lens group which is made up of a single positive lens in the vicinity of the light valve in the airspace between the second lens group and the light valve.

In addition, FIG. 17 shows an external appearance of a projector which utilizes the lens system of the invention. As is shown in FIG. 17, a projector 10 is formed substantially into a rectangular parallelepiped shape, and the projector 10 has a lens cover 19 which covers a projection opening provided at an end of a front plate 12 which makes up a main body housing, and a plurality of vent holes 18 are provided in the front plate 12. In addition, although the illustration thereof is omitted in FIG. 17, a key/indicator section is provided on an upper plate 11 which also makes up the main body housing, and this key/indicator section includes keys and indicators such as a power supply switch, a power indicator lamp which informs that the power supply is on or off, a lamp switch key for turning on a lamp of a light source unit, an indicator lamp which indicates that the lamp is turned on and an overheat indicator lamp which informs that the light source unit is under overheat when it actually happens. Furthermore, provided on a back plate, which is not shown, of the main body housing are an input/output connector section where USB terminals, D-SUB terminals for image signal input, S terminals, RCA terminals and the like are provided and an Ir receiving section where control signals from a power supply adapter plug and a remote controller are received. In addition, a plurality of vent holes 18 are provided in a right-hand side plate, not shown, which is a side plate of the main body housing, and a left-hand side plate 15, which is a side plate shown in FIG. 17, and a projection lens which adopts the lens system that has been described heretofore is installed inside the left-hand side plate 15 therealong.

Embodiment 1

A numerical example of a first embodiment of a lens system of the invention is shown in Table 1. In addition, FIG. 1 is a drawing showing the configuration of the lens system, and FIG. 2 is a drawing showing aberrations thereof. In the following tables and drawings, f denotes a focal length of the whole lens system, F_(no) an F number, and 2ω total angle of view of the lens system. In addition, r denotes a radius of curvature, d a lens element thickness or spacing between lens elements, n_(d) a refractive index relative to the d line, and ν_(d) Abbe number at the d line. In the drawings showing spherical aberration of the drawings showing the aberrations, CA1, CA2, CA3 denote aberration curves at wavelengths of CA1=550.0 nm, CA2=435.8 nm and CA3=640.0 nm, and S.C denotes a sine condition. In the drawing showing astigmatism, S denotes sagital, and M denotes meridional. In addition, throughout the tables, unless otherwise described, the wavelength used in calculation of numerical values is CA1=550.0 nm, and the drawings showing the aberrations show aberrations which result with an object distance of 1700 mm which is used in many cases for evaluation of a projection lens.

TABLE 1 f 16.00 F_(no) 2.29 2ω 65.73 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 57.250 3.00 1.51680 64.20 2 112 ∞ 1.16 — — 3 113 27.311 1.20 1.80518 25.46 4 114 12.138 4.07 — — 5 115 92.547 1.20 1.80610 40.73 6 116 17.045 3.74 — — 7 121 −36.388 1.20 1.80420 46.50 8 122(123) 30.503 4.98 1.67270 32.17 9 124 −18.820 2.58 — — 10 125 −13.913 2.25 1.72916 54.67 11 126 −23.849 0.20 — — 12 127 −52.994 5.00 1.80518 25.46 13 128 −33.940 0.34 — — 14 129 −54.078 3.38 1.48749 70.45 15 1210 −21.051 27.52 — — 16 211 −103.355 3.04 1.69700 48.51 17 212 −29.361 1.29 — — 18 213 65.511 4.96 1.49700 81.61 19 214(215) −20.364 4.41 1.80420 46.50 20 216 55.620 5.13 — — 21 221 42.848 6.00 1.51680 64.20 22 222 −33.313 0.20 — — 23 223 40.041 1.40 1.80100 34.97 24 224(225) 16.566 6.00 1.48749 70.45 25 226 −323.311 34.00 — — 26 301 −100.000 3.00 1.77250 49.60 27 302 −44.000 0.66 — — 28 C01 ∞ 3.00 1.48745 64.84 29 C02 ∞ — — —

Embodiment 2

A numerical example of a second embodiment of a lens system of the invention is shown in Table 2. In addition, FIG. 3 is a drawing showing the configuration of the lens system, and FIG. 4 is a drawing showing aberrations thereof.

TABLE 2 f 15.93 F_(no) 2.29 2ω 65.96 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 60.251 3.00 1.51680 64.20 2 112 −1021.642 1.15 — — 3 113 27.757 1.20 1.80518 25.46 4 114 12.265 3.80 — — 5 115 67.545 1.20 1.80518 25.46 6 116 16.521 4.80 — — 7 121 −36.351 3.20 1.80420 46.50 8 122(123) 24.151 5.90 1.76182 26.61 9 124 −21.573 1.72 — — 10 125 −13.602 1.40 1.51680 64.20 11 126 −28.595 0.10 — — 12 127 −48.659 6.50 1.51680 64.20 13 128 −17.943 25.44 — — 14 211 3189.749 5.50 1.51680 64.20 15 212 −32.148 3.23 — — 16 213 68.321 6.41 1.51680 64.20 17 214(215) −18.692 3.50 1.80420 46.50 18 216 50.349 1.99 — — 19 221 42.917 6.50 1.51680 64.20 20 222 −26.489 0.20 — — 21 223 42.375 1.40 1.80610 33.27 22 224(225) 16.536 6.04 1.51680 64.20 23 226 −323.311 34.00 — — 24 301 −100.000 3.00 1.77250 49.60 25 302 −44.000 0.66 — — 26 C01 ∞ 3.00 1.48745 64.84 27 C02 ∞ — — —

Embodiment 3

A numerical example of a third embodiment of a lens system of the invention is shown in Table 3. In addition, FIG. 5 is a drawing showing the configuration of the lens system, and FIG. 6 is a drawing showing aberrations thereof.

TABLE 3 f 15.93 F_(no) 2.29 2ω 65.95 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 70.489 3.00 1.51680 64.20 2 112 −260.316 1.15 — — 3 113 36.942 1.20 1.80518 25.46 4 114 12.782 3.82 — — 5 115 154.147 1.20 1.80518 25.46 6 116 18.891 3.62 — — 7 121 −32.477 1.73 1.80420 46.50 8 122(123) 32.490 5.00 1.74077 27.76 9 124 −18.951 1.42 — — 10 125 −13.373 1.40 1.71736 29.50 11 126 −22.882 0.10 — — 12 127 −48.045 5.70 1.71736 29.50 13 128 −19.558 30.47 — — 14 211 232.465 4.89 1.51742 52.16 15 212 −27.481 1.39 — — 16 213 118.905 6.00 1.48749 70.45 17 214(215) −18.003 5.00 1.80420 46.50 18 216 54.195 2.95 — — 19 221 39.606 6.20 1.48749 70.45 20 222 −27.838 0.20 — — 21 223 34.934 1.40 1.80610 33.27 22 224(225) 15.755 6.30 1.48749 70.45 23 226 −323.311 34.00 — — 24 301 −100.000 3.00 1.77250 49.60 25 302 −44.000 0.66 — — 26 C01 ∞ 3.00 1.48745 64.84 27 C02 ∞ — — —

Embodiment 4

A numerical example of a fourth embodiment of a lens system of the invention is shown in Table 4. In addition, FIG. 7 is a drawing showing the configuration of the lens system, and FIG. 8 is a drawing showing aberrations thereof.

TABLE 4 f 15.93 F_(no) 2.29 2ω 66.01 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 53.769 3.20 1.51680 64.20 2 112 −1614.044 0.29 — — 3 113 26.752 1.66 1.80420 46.50 4 114 12.765 3.65 — — 5 115 68.006 1.20 1.80420 46.50 6 116 15.268 3.66 — — 7 121 −48.194 1.20 1.80420 46.50 8 122(123) 34.131 7.68 1.69895 30.05 9 124 −21.387 1.39 — — 10 125 −14.551 3.00 1.80518 25.46 11 126 −23.922 1.63 — — 12 127 −45.447 4.89 1.70200 40.20 13 128 −21.750 27.56 — — 14 211 −232.786 5.20 1.51760 63.50 15 212 −27.179 1.76 — — 16 213 131.090 5.03 1.48749 70.45 17 214(215) −19.074 6.04 1.80450 39.64 18 216 65.793 0.51 — — 19 221 41.955 6.71 1.62230 53.17 20 222 −35.659 0.54 — — 21 223 41.197 1.40 1.80420 46.50 22 224(225) 16.305 6.14 1.51680 64.20 23 226 −319.835 34.00 — — 24 301 −100.000 3.00 1.76182 26.61 25 302 −44.000 0.66 — — 26 C01 ∞ 3.00 1.48745 64.84 27 C02 ∞ — — —

Embodiment 5

A numerical example of a fifth embodiment of a lens system of the invention is shown in Table 5. In addition, FIG. 9 is a drawing showing the configuration of the lens system, and FIG. 10 is a drawing showing aberrations thereof.

TABLE 5 f 15.93 F_(no) 2.29 2ω 66.02 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 56.346 3.20 1.48749 70.45 2 112 −629.938 0.20 — — 3 113 32.213 1.20 1.80420 46.50 4 114 14.319 3.75 — — 5 115 95.988 1.20 1.80420 46.50 6 116 16.964 3.49 — — 7 121 −92.076 1.20 1.80420 46.50 8 122(123) 25.965 6.90 1.67270 32.17 9 124 −22.386 2.69 — — 10 125 −15.853 2.80 1.80518 25.46 11 126 −25.181 2.68 — — 12 127 −44.259 5.53 1.64769 33.84 13 128 −22.308 23.88 — — 14 211 −208.291 5.20 1.48749 70.45 15 212 −27.810 4.78 — — 16 213 112.109 4.84 1.48749 70.45 17 214(215) −19.103 8.14 1.80610 33.27 18 216 74.225 0.75 — — 19 221 47.892 6.60 1.58144 40.89 20 222 −32.456 0.20 — — 21 223 36.851 1.40 1.80420 46.50 22 224(225) 17.133 5.95 1.51680 64.20 23 226 −387.173 34.66 — — 24 C01 ∞ 3.00 1.48745 64.84 25 C02 ∞ — — —

Embodiment 6

A numerical example of a sixth embodiment of a lens system of the invention is shown in Table 6. In addition, FIG. 11 is a drawing showing the configuration of the lens system, and FIG. 12 is a drawing showing aberrations thereof.

TABLE 6 f 15.93 F_(no) 2.29 2ω 66.01 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 52.751 3.18 1.48749 70.45 2 112 ∞ 0.20 — — 3 113 33.043 1.24 1.80420 46.50 4 114 13.647 4.05 — — 5 115 −95.922 1.20 1.80420 46.50 6 116 16.420 3.60 — — 7 121 −85.483 1.20 1.80420 46.50 8 122(123) 62.591 4.61 1.63980 34.57 9 124 −19.695 2.43 — — 10 125 −16.204 2.77 1.71300 53.94 11 126 −22.105 6.45 — — 12 127 −21.019 5.90 1.74950 35.04 13 128 −19.087 31.21 — — 14 211 −42.812 5.20 1.48749 70.45 15 212 −23.589 0.20 — — 16 213 78.117 5.80 1.48749 70.45 17 214(215) −20.355 1.40 1.78590 43.93 18 216 ∞ 0.20 — — 19 221 32.412 4.68 1.51680 64.20 20 222 −58.965 2.62 — — 21 223 78.293 3.00 1.80420 46.50 22 224(225) 16.655 5.48 1.51760 63.50 23 226 ∞ 34.00 — — 24 301 35.000 20.00 1.51680 64.20 25 302 ∞ 0.66 — — 26 C01 ∞ 3.00 1.48745 64.84 27 C02 ∞ — — —

Embodiment 7

A numerical example of a seventh embodiment of a lens system of the invention is shown in Table 7. In addition, FIG. 13 is a drawing showing the configuration of the lens system, and FIG. 14 is a drawing showing aberrations thereof.

TABLE 7 f 15.93 F_(no) 2.29 2ω 65.96 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 57.931 3.00 1.51680 64.20 2 112 −4318.776 1.15 — — 3 113 26.555 1.20 1.80518 25.46 4 114 12.082 3.97 — — 5 115 79.183 1.20 1.80518 25.46 6 116 16.478 3.45 — — 7 121 −52.074 1.80 1.80420 46.50 8 122(123) 32.306 5.00 1.71736 29.50 9 124 −18.472 0.95 — — 10 125 −14.276 2.60 1.51680 64.20 11 126 104.858 1.60 — — 12 127 91.621 5.00 1.58144 40.89 13 128 −19.414 0.20 — — 14 129 −25.821 3.50 1.51680 64.20 15 1210 −31.898 26.11 — — 16 211 316.605 5.00 1.51680 64.20 17 212 −31.354 1.95 — — 18 213 74.983 6.00 1.51680 64.20 19 214(215) −19.202 5.00 1.80420 46.50 20 216 51.662 1.83 — — 21 221 39.494 6.00 1.51680 64.20 22 222 −29.476 0.20 — — 23 223 42.479 1.40 1.80610 33.27 24 224(225) 16.297 6.10 1.51680 64.20 25 226 −323.311 34.00 — — 26 301 −100.000 3.00 1.77250 49.60 27 302 −44.000 0.66 — — 28 C01 ∞ 3.00 1.48745 64.84 29 C02 ∞ — — —

Embodiment 8

A numerical example of an eighth embodiment of a lens system of the invention is shown in Table 8. In addition, FIG. 15 is a drawing showing the configuration of the lens system, and FIG. 16 is a drawing showing aberrations thereof.

TABLE 8 f 15.93 F_(no) 2.29 2ω 65.98 Serial Surface numbers numbers r d n_(d) ν_(d) 1 111 58.957 3.04 1.51680 64.20 2 112 −988.890 1.14 — — 3 113 34.363 1.20 1.80518 25.46 4 114 11.831 4.01 — — 5 115 79.541 1.20 1.80518 25.46 6 116 17.857 3.53 — — 7 121 −41.517 1.60 1.80420 46.50 8 122(123) 40.347 5.00 1.71736 29.50 9 124 −17.256 0.97 — — 10 125 −13.865 1.40 1.51680 64.20 11 126 92.438 0.10 — — 12 127 60.711 5.10 1.58144 40.89 13 128 −20.652 2.00 — — 14 129 −19.416 3.50 1.51680 64.20 15 1210 −18.591 25.98 — — 16 211 −2007.690 5.00 1.51680 64.20 17 212 −33.991 2.40 — — 18 213 57.339 6.00 1.51680 64.20 19 214(215) −19.047 5.00 1.80420 46.50 20 216 45.560 2.20 — — 21 221 41.552 6.00 1.51680 64.20 22 222 −26.131 0.20 — — 23 223 41.611 1.41 1.80610 33.27 24 224(225) 16.337 6.10 1.51680 64.20 25 226 −323.311 34.00 — — 26 301 −100.000 3.00 1.77250 49.60 27 302 −44.000 0.66 — — 28 C01 ∞ 3.00 1.48745 64.84 29 C02 ∞ — — —

Next, values corresponding to the conditional expression (1) to conditional expression (19) in the first embodiment to the eight embodiment are shown in Table 9 altogether.

TABLE 9 Conditional Embodiment Expression 1 2 3 4 5 6 7 8  (1) −0.24 −0.32 −0.32 −0.29 −0.29 −0.34 −0.35 −0.29  (2) 0.41 0.42 0.44 0.43 0.43 0.40 0.43 0.42  (3) −1.07 −1.02 −1.13 −0.99 −0.97 −1.06 −1.05 −1.14  (4) −0.03 −0.05 −0.04 −0.09 −0.07 0.04 −0.02 −0.07  (5) 0.48 0.54 0.53 0.56 0.57 0.30 0.52 0.55  (6) 2.14 2.13 1.84 2.10 2.19 2.31 2.17 2.12  (7) 0.76 0.77 0.80 0.80 0.90 0.86 0.76 0.74  (8) 1.71 1.71 1.71 1.71 1.70 1.70 1.71 1.71  (9) −1.32 −1.13 −1.23 −1.37 −1.40 −1.20 −2.00 −1.17 (10) −7.89 −25.08 −18.91 −12.56 −13.98 −27.02 −23.11 −10.49 (11) 1.77 1.66 1.76 1.80 1.80 1.76 1.61 1.66 (12) 0.21 0.29 0.30 0.30 0.32 0.30 0.29 0.29 (13) 18.56 17.70 14.81 27.34 37.18 26.52 17.70 17.70 (14) −0.89 −0.97 −0.98 −0.90 −0.88 −0.62 −0.95 −0.99 (15) −0.45 −0.47 −0.23 −0.21 −0.25 −0.30 −0.42 −0.59 (16) 0.30 0.29 0.32 0.23 0.26 0.29 0.29 0.29 (17) 32.36 30.93 37.18 12.19 6.05 17.35 30.93 30.93 (18) −0.44 −0.46 −0.44 −0.46 −0.39 −0.60 −0.48 −0.47 (19) −1.29 −1.62 −1.42 −1.18 −1.48 −0.55 −1.34 −1.59

As is obvious from Table 9, the numerical values related to the respective examples of the first to eighth embodiments satisfy the conditional expressions (1) to (19), and as is obvious from the aberration drawings of the respective embodiments, the respective aberrations were corrected properly. 

1. A lens system comprising, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, the first lens group being configured to comprise, sequentially in that order from a magnifying side, a first sub-lens group or 1a lens group having a negative refractive power as a whole and a second sub-lens group or 1b lens group having a positive refractive power as a whole, the second lens group being configured to comprise, sequentially in that order from a magnifying side, a first sub-lens group or 2a lens group having a negative refractive power as a whole and a second sub-lens group or 2b lens group having a positive refractive power as a whole, wherein, the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the 1a lens group, the following conditional expression (4) is satisfied with respect to a power set for the 2a lens group, the following conditional expression (5) is satisfied with respect to a power set for the 2b lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group; −0.5≦f/f _(I)≦−0.1  (1) 0.3≦h _(IE) /h _(IX)≦0.5  (2) −1.5≦f/f _(Ia)≦−0.5  (3) −0.3≦f/f _(IIa)≦0.3  (4) 0.15≦f/f _(IIb)≦0.7  (5) 1.6≦T _(I) /f≦2.6  (6) where, f: Composite focal length of the whole lens system; f_(I): Composite focal length of the first lens group; h_(IE): Height of paraxial ray incident on to a magnifying-side surface of a lens element which is disposed outermost on a magnifying side of the first lens group; h_(IX): Height of paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f_(Ia): Composite focal length of the 1a lens group which makes up the first lens group; f_(IIa): Composite focal length of the 2a lens group which makes up the second lens group; f_(IIb): Composite focal length of the 2b lens group which makes up the second lens group; and T_(I): Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.
 2. A lens system as set forth in claim 1, wherein the 1a lens group, which makes up the first lens group, is configured to comprise three lens elements, sequentially in that order from a magnifying side, a lens element which has a positive refractive power (hereinafter, referred to as a positive lens element), a lens element which is formed into a meniscus shape which is convex on a magnifying side thereof and which has a negative refractive power (hereinafter, referred to as a negative lens element), and a negative lens element, and wherein the following conditional expression (7) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1a lens group, and the following conditional expression (8) is satisfied with respect to the refractive index of a glass material used for each of the lens elements which make up the 1a lens group; 0.6≦R _(Ia4) /f≦1.1  (7) 1.65≦N_(Ia)  (8) where, R_(Ia4): Radius of curvature of the contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1a lens group; and N_(Ia): Mean value of refractive indices of the lens elements which make up the 1a lens group relative to the d line.
 3. A lens system as set forth in claim 1, wherein the 1b lens group is configured to comprise four lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof, or is configured to comprise five lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, a positive lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof or a negative lens element which has a meniscus shape which is convex on a contracting side thereof, and wherein the following conditional expression (9) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the contracting side of the 1b lens group, the following conditional expression (10) is satisfied with respect to the dispersion properties of a glass material used for each lens element which makes up the 1b lens group, and the following conditional expression (11) is satisfied with respect to the refractive index of a glass material used for each of the negative lens elements which make up the 1b lens group; −2.8≦R _(IbL) /f≦−1.0  (9) V _(IbP) −V _(IbN)≦−5  (10) 1.58≦N_(IbN)  (11) where, R_(IbL): Radius of curvature of the contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 1b lens group; V_(IbP): Mean value of Abbe numbers of the positive lens elements which make up the 1b lens group; V_(IbN): Mean value of Abbe numbers of the negative lens elements which make up the 1b lens group; and N_(IbN): Mean value of the refractive indices of the negative lens elements which make up the 1b lens group relative the d line.
 4. A lens system as set forth in claim 1, wherein the 2a lens group is configured to comprise three lens elements, that is, a positive lens element, a positive lens element and a negative lens element, and wherein the following conditional expressions (12), (13) are satisfied, respectively, with respect to the refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2a lens group, the following conditional expression (14) is satisfied with respect to a power set for the negative lens element which is disposed outermost on the contracting side of the 2a lens group, and the following conditional expression (15) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2a lens group and the shape of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the same lens group; 0.15≦N _(IIaN) −N _(IIaP)  (12) 10≦V _(IIaP) −V _(IIaN)  (13) −1.5≦f/f _(IIaN)≦−0.4  (14) −0.75≦R _(IIa2) /R _(IIa3)≦−0.1  (15) where, N_(IIaN): Mean value of refractive indices of the negative lens element which makes up the 2a lens group relative to the d line; N_(IIaP): Mean value of refractive indices of the positive lens elements which make up the 2a lens group relative to the d line; V_(IIaP): Mean value of Abbe numbers of the positive lens elements which make up the 2a lens group; V_(IIaN): Mean value of Abbe numbers of the negative lens elements which make up the 2a lens group; f_(IIaN): Focal length of the lens element which is disposed outermost on the contracting side of the lens elements which make up the 2a lens group; R_(IIa2): Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the lens elements which make up the 2a lens group; and R_(IIa3): Radius of curvature of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the lens elements which make up the 2a lens group.
 5. A lens system as set forth in claim 1, wherein the 2b lens group is configured to comprise three lens elements in total, that is, a positive lens element, a negative lens element, and a positive lens element, and wherein the following conditional expressions (16), (17) are satisfied, respectively, with respect to refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2b lens group, the following conditional expression (18) is satisfied with respect to a power set for the lens element which is disposed second outermost from the magnifying side of the 2b lens group, and the following conditional expression (19) is satisfied with respect to the shape of a magnifying-side surface and the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group; 0.15≦N _(IIbN) −N _(IIbP)  (16) 0≦V _(IIbP) −V _(IIbN)  (17) −1.0≦f/f _(IIb2)≦−0.2  (18) −2.0≦R _(IIb1) /R _(IIb2)≦−0.3  (19) where, N_(IIbN): Mean value of refractive indices of the negative lens element which makes up the 2b lens group relative to the d line; N_(IIbP): Mean value of refractive indices of the positive lens elements which make up the 2b lens group relative to the d line; V_(IIbP): Mean value of Abbe numbers of the positive lens elements which make up the 2b lens group; V_(IIbN): Mean value of Abbe numbers of the negative lens element which makes up the 2b lens group; f_(IIb2): Focal length of the lens element which lies second outermost from the magnifying side of the lens elements which make up the 2b lens group; R_(IIb1): Radius of curvature of a magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group; and R_(IIb2): Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2b lens group.
 6. A lens system as set forth in claim 1, wherein an adjustment of a focal point of the whole lens system is implemented by shifting the second lens group in an optical direction thereof.
 7. A lens system as set forth in claim 2, wherein an adjustment of a focal point of the whole lens system is implemented by shifting the second lens group in an optical direction thereof.
 8. A lens system as set forth in claim 3, wherein an adjustment of a focal point of the whole lens system is implemented by shifting the second lens group in an optical direction thereof.
 9. A lens system as set forth in claim 4, wherein an adjustment of a focal point of the whole lens system is implemented by shifting the second lens group in an optical direction thereof.
 10. A lens system as set forth in claim 5, wherein an adjustment of a focal point of the whole lens system is implemented by shifting the second lens group in an optical direction thereof.
 11. A lens system as set forth in claim 1, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 12. A lens system as set forth in claim 2, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 13. A lens system as set forth in claim 3, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 14. A lens system as set forth in claim 4, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 15. A lens system as set forth in claim 5, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 16. A lens system as set forth in claim 6, wherein a third lens group comprising a single positive lens is provided in the vicinity of a light valve in an airspace between the second lens group and the light valve.
 17. A projector installing the lens system set forth in claim
 1. 18. A projector installing the lens system set forth in claim
 2. 19. A projector installing the lens system set forth in claim
 3. 20. A projector installing the lens system set forth in claim
 4. 